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package org.openjdk.jmh.samples;


import org.openjdk.jmh.annotations.*;

import org.openjdk.jmh.runner.Runner;

import org.openjdk.jmh.runner.RunnerException;

import org.openjdk.jmh.runner.options.Options;

import org.openjdk.jmh.runner.options.OptionsBuilder;


import java.util.concurrent.TimeUnit;


@State(Scope.Thread)

@BenchmarkMode(Mode.AverageTime)

@OutputTimeUnit(TimeUnit.NANOSECONDS)

public class JMHSample_12_Forking {


 /*

  * JVMs are notoriously good at profile-guided optimizations. This is bad

  * for benchmarks, because different tests can mix their profiles together,

  * and then render the "uniformly bad" code for every test. Forking (running

  * in a separate process) each test can help to evade this issue.

  *

  * JMH will fork the tests by default.

  */


 /*

  * Suppose we have this simple counter interface, and two implementations.

  * Even though those are semantically the same, from the JVM standpoint,

  * those are distinct classes.

  */


 public interface Counter {

  int inc();

 }


 public static class Counter1 implements Counter {

  private int x;


  @Override

  public int inc() {

   return x++;

  }

 }


 public static class Counter2 implements Counter {

  private int x;


  @Override

  public int inc() {

   return x++;

  }

 }


 /*

  * And this is how we measure it.

  * Note this is susceptible for same issue with loops we mention in previous examples.

  */


 public int measure(Counter c) {

  int s = 0;

  for (int i = 0; i < 10; i++) {

   s += c.inc();

  }

  return s;

 }


 /*

  * These are two counters.

  */

 Counter c1 = new Counter1();

 Counter c2 = new Counter2();


 /*

  * We first measure the Counter1 alone...

  * Fork(0) helps to run in the same JVM.

  */


 @Benchmark

 @Fork(0)

 public int measure_1_c1() {

  return measure(c1);

 }


 /*

  * Then Counter2...

  */


 @Benchmark

 @Fork(0)

 public int measure_2_c2() {

  return measure(c2);

 }


 /*

  * Then Counter1 again...

  */


 @Benchmark

 @Fork(0)

 public int measure_3_c1_again() {

  return measure(c1);

 }


 /*

  * These two tests have explicit @Fork annotation.

  * JMH takes this annotation as the request to run the test in the forked JVM.

  * It's even simpler to force this behavior for all the tests via the command

  * line option "-f". The forking is default, but we still use the annotation

  * for the consistency.

  *

  * This is the test for Counter1.

  */


 @Benchmark

 @Fork(1)

 public int measure_4_forked_c1() {

  return measure(c1);

 }


 /*

  * ...and this is the test for Counter2.

  */


 @Benchmark

 @Fork(1)

 public int measure_5_forked_c2() {

  return measure(c2);

 }


 /*

  * ============================== HOW TO RUN THIS TEST: ====================================

  *

  * Note that C1 is faster, C2 is slower, but the C1 is slow again! This is because

  * the profiles for C1 and C2 had merged together. Notice how flawless the measurement

  * is for forked runs.

  *

  * You can run this test:

  *

  * a) Via the command line:

  *    $ mvn clean install

  *    $ java -jar target/benchmarks.jar JMHSample_12

  *

  * b) Via the Java API:

  *    (see the JMH homepage for possible caveats when running from IDE:

  *      http://openjdk.java.net/projects/code-tools/jmh/)

  */


 public static void main(String[] args) throws RunnerException {

  Options opt = new OptionsBuilder()

          .include(JMHSample_12_Forking.class.getSimpleName())

          .build();


  new Runner(opt).run();

 }
// 本例介绍了 @Fork 注解，@Fork 可以指定代码运行时是否需要 fork 出一个JVM进程，如果在同一个JVM中测试则会相互影响，一般fork进程设置为1。

}